The present invention relates to a method for designing and manufacturing a micromechanical device, in particular a micromechanical oscillating mirror.
Although in principle applicable to arbitrary micromechanical devices, the present invention as well as its underlying problem are explained with respect to a micromechanical oscillating mirror.
Micromechanical oscillating mirrors are used, for example, as switching elements in optical transmission engineering or as scanning element for deflecting a laser beam for bar-code recognition, for room monitoring, or as a marking instrument.
The present invention is based on the problem that the micromechanical conventional oscillating mirrors are short-lived and difficult to manufacture. In particular, temperature problems and problems due to mechanical stresses exist with the materials used such as polysilicon. By using low-stress electroplating layers, it is possible, in particular for mirror surfaces to be manufactured without curvature.
The designing and manufacturing method according to the present invention has the advantage over conventional design approaches that the resulting micromechanical device is stress- and temperature-compensated so that both freedom in the choice of material and in the selection of the operating temperature, which is typically in the range of xe2x88x9240xc2x0 C. to +130xc2x0 C., exists.
It is the basic idea of the present invention that the micromechanical device electrodeposited on the unmasked region of the adhesion layer using the xe2x80x9cadditive techniquexe2x80x9d is supported in the anchoring region and tiltable about at least one axis or able to execute torsional vibrations subsequent to the removal of the sacrificial layer. In the proposed designs, the advantages of the additive technique can be fully exhausted.
The additive technique makes it possible to reduce the size of the previous micromechanical design approaches and, in connection with that, to reduce the price and develop new possibilities for use. The designing and manufacturing method according to the present invention thus provides cost-effective, reliable and long-lived micromechanical devices. In particular, the additive technique allows freely movable metal patterns to be produced on an arbitrary substrate such as a silicon substrate, glass substrate, or a ceramic substrate.
In addition, the additive technique allows large, unperforated surfaces to be bared so that massive mirror surfaces having dimensions up to several millimeters can be manufactured. As a single-layer electroplating process, the technique is cost-effective and can be controlled well. A multilayer electroplating process can be carried out, as well, for example, for manufacturing the anchoring regions and the mirror surface or the suspensions separately. Large tilting angles can be attained by correspondingly thick sacrificial layers.
According to an example embodiment, a metallic connection pad, e.g. of a circuit integrated in the substrate is provided as anchoring region. Both a manufacture as discrete device and a manufacture in a form that is integrated in a service connection are possible. If the micromechanical device is integrated on an integrated circuit, the metallization of the integrated circuit can advantageously be used for anchoring.
According to a further preferred embodiment, a first photoresist layer having a thickness of several microns is formed as sacrificial layer. The photoresist can easily be removed in an isotropic etching process. When using a polymer sacrificial layer, the distance of the mirror element from the substrate can be adjusted very accurately, distances from several microns to approximately 150 xcexcm may be achieved.
In another example embodiment, the first photoresist layer is patterned photolithographically for leaving bare the anchoring region.
According to a further example embodiment, the adhesion layer is sputtered.
In a further example embodiment, the adhesion layer is a conductive layer of Cuxe2x80x94Cr having a thickness of several nanometers. The chromium serves as adhesion layer toward the underlying photoresist; the copper serves as a starting layer for the subsequent electrodeposition. Other adhesion layers, such as Crxe2x80x94Au, etc., are, of course, also possible.
According to a further example embodiment, the mask is formed on the adhesion layer by the following steps: forming a second photoresist layer on the adhesion layer; forming a silicon dioxide layer on the second photoresist layer; patterning a third photoresist layer photolitographically; and plasma etching the silicon dioxide layer for forming a hard mask for the second photoresist layer; and etching the second photoresist layer masked by the patterned silicon dioxide layer down to the adhesion layer. In this context, the second photoresist layer is used as polymer negative matrix for the electrodeposition.
In another preferred embodiment, a nickel layer or a nickel-cobalt layer is deposited as electroplating layer. Layers of that kind can be manufactured free of stress, evenly, and with a good reflectivity.
According to a further example embodiment, the sacrificial layer in the form of the first photoresist layer, the polymer mold formed by the second photoresist layer, and the adhesion layer are removed subsequent to the deposition of the electroplating layer.
In a further example embodiment, the micromechanical device is an oscillating mirror which is designed in such a manner that it can execute torsional vibrations about at least one axis. The oscillating mirror can be operated as a simple tilting mirror as well as in resonance as scanning mirror when using a thicker sacrificial layer. The oscillating mirrors can be designed in such a manner that they are tiltable in one, two or as many as desired directions.
According to another example embodiment, a counter-electrode is provided on the substrate under the mirror surface.
According to further example embodiment, the oscillating mirror is designed such that it can execute torsional vibrations about four or more axes.
In a further example embodiment, the oscillating mirror is designed such that it is suspended on a surrounding frame which is anchored in the anchoring region. Thus, an uninterrupted or non-cutout mirror region can be achieved.
According to another example embodiment, the oscillating mirror is designed such that the anchoring region is provided in a cutout of the mirror surface.